Project Summary Neurons in the mammalian brain possess elaborate tree-like structures termed dendrites. These dendritic branches with distinctive morphological features compartmentalize and shape synaptic inputs, which eventually propagate to the soma to form the action-potential output (AP) ? the unit currency of information transfer in the brain. Unravelling how dendrites transform complex synaptic inputs into AP output, shape population-level brain activity, and drive sensory input fundamental to our understanding of neural circuit mechanisms and brain function. Of particular interest are the distal dendrites of layer 5 pyramidal neurons in the barrel cortex, whose somatic output transfers sensorimotor information to a myriad of brain regions. These deep-layer pyramidal neurons extend their dendrites vertically into the most superficial layer of the cortex where they integrate extensive inputs from diverse brain regions, and exhibit a myriad of regenerative feedback events (E.g.: NMDA, Calcium, Sodium spikes), which are believed to be fundamental to controlling the overall input-output-gain of the neuron. However, the spatio-temporal dynamics of dendritic activity, and the overall relationship between dendritic and somatic gain in vivo remains unknown. The small size of these dendrites (~2 m in diameter) and their location (<100 m from the surface) has rendered conventional whole-cell electrophysiology infeasible ? the current gold-standard; while the proposed alternative approaches, such as calcium imaging, lack temporal resolution to report fast sub-threshold membrane dynamics that underlie neural computation. Here, we aim to map the electrical dynamics of distal apical dendrites in the somatosensory cortex of awake behaving mice using a flexible and transparent vertical nanoelectrode platform termed ?The Nanoneedle Net?. The Net will comprise of 256 channels with 128 planar electrodes and 128 vertical needles. Each needle will be 40-60 m in height, ~100 nm in tip diameter, and lipid-coated to facilitate seamless penetration into a membrane. Our custom fabricated array (electrode pitch of 20 m) once placed on the surface of the brain will allow the needles to penetrate <60 m deep and form a tight electrical seal with distal dendritic branches. Readout will be accomplished through heavily multiplexed low noise custom CMOS amplifiers. To corroborate the origins of both planar surface recordings and nanoneedle dendritic recordings in vivo, we will combine conventional intra- and extracellular ground-truth electrophysiology, two-photon calcium imaging, optogenetics, spike sorting using template matching, and whisker touch. Ultimately, this platform will not only allow us to establish the computational rules by which distal dendrites shape cortical output during active sensation, but provide a universal method to probe dendritic integration in the living brain.